Immobilization of high level nuclear waste in glass is the most extensively studied solid waste form. The usual practice is to dissolve waste oxides and molten glass at high temperatures, that is, greater than 1000.degree. C. In order to achieve homogenization of the mixture in a reasonable time, glass forming systems are selected that have melt viscosities on the order of 20 poises at melt temperatures between 1000.degree. C. and 1200.degree. C. These constraints are best met by borosilicate glass compositions. Although possessing good properties from the viewpoint of waste oxide solubility and glass forming properties, borosilicate waste glasses are lacking in chemical durability. Conversely, glass compositions having excellent chemical durability, such as natural obsidians and nepheline syenite, require such high forming temperatures when made by conventional processing methods that they have been largely eliminated from consideration as a solid waste form.
Even the most intimate mechanical mixing of oxide waste and glass frit (or glass forming constituents) cannot circumvent the non-equilibrium events that can occur during the homogenization process at high temperatures. For example, sublimation of the uncombined oxide is more likely to produce volatilization than is vaporization from a homogenized melt. Specific reactions between the oxide waste and the glass frit to give metastable high volatility melts is more likely to lead to foaming during homogenization than in the final melt composition. Phase separation by density difference creates a degree of heterogeneity that cannot be easily reversed by mixing the viscous melt. Finally, prolonged mixing of submicron powder and glass frit causes additional phase separation due to dusting before the mixture has an opportunity to melt.